Particle beam microscope and method for operating the particle beam microscope

ABSTRACT

A method for operating a particle beam microscope comprising detecting light rays or particles which emanate from a structure, wherein the structure comprises at least one of: at least a portion of a surface of an object and at least a portion of a surface of an object holder of the particle beam microscope; generating a surface model of the structure depending on the at least one of the detected light rays and the particles; determining a position and an orientation of the surface model of the structure relative to the object region; determining a measurement location relative to the surface model of the structure; and positioning the object depending on the generated surface model of the structure, depending on the determined position and orientation of the surface model of the structure, and depending on the determined measurement location.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims priority of German Patent Application No.10 2010 046 902.5, filed Sep. 29, 2010 in Germany, entitled“Partikeistrahlmikroskop and Verfahren zum Betreiben hierzu”, and ofGerman Patent Application No. 10 2011 103 997.3, filed. Jun. 10, 2011 inGermany, entitled “Partikelstrahlmikroskop und Verfahren sum Betreibenhierzu”, and of U.S. patent application Ser. No. 13/029,998, entitled“Method of Operating a Scanning Electron Microscope”: the contents ofthese documents are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a particle beam microscope and a methodfor operating a particle beam microscope. More specifically, the presentinvention relates to an electron microscope, such as a scanning electronmicroscope and a method for operating a scanning electron microscope.

BACKGROUND

When samples are imaged or processed with a particle beam microscope,such as an electron microscope, they are usually kept in a vacuumenvironment in the specimen chamber. The specimen chamber is evacuatedby a vacuum pump. Typically, measurements with scanning electronmicroscopes are conducted at a vacuum level in the specimen chamber in arange of about high vacuum and about 22.5 Torr. The specimen chamber istherefore designed as a vacuum vessel, having solid walls and flanges,such that the leaking rates of atmospheric leaks can be kept as low aspossible. Hence, the vacuum vessel usually does not have windows, whichare large enough to allow a user to control the positioning of theobject in front of the objective lens by visual observation.

Typically, the positioning of the sample is monitored by a COD-camera,which is arranged within the specimen chamber. The camera acquires avideo image from the sample and the objective lens, which is displayedon a display. By looking at the video image, the user can observe thepositioning process in real time and control the positioning of thesample via control signals, which are transmitted to a positioningdevice.

However, the displayed video image provides the user only with a twodimensional image from the interior of the specimen chamber, such thatit is complicated to accurately position the object relative to theobject lens. Furthermore, the viewing angle of the COD-camera forobserving the object surface is typically obstructed by the objectivelens and detectors, especially when the object is located close to theobjective lens. Hence, the user quite often is not able to determine,which part of the sample is irradiated by the electron beam.

Beside the objective lens, there are typically also further componentsarranged in the interior of the specimen chamber, which may obstruct theview to the sample during a positioning process. Examples of suchcomponents are detectors, gas injection systems and manipulators. Thesecomponents may also collide with the sample during a positioningprocess.

Conducting the positioning is even more complicated when a number ofobjects, in particular objects having a complex geometry, are attachedto the object holder for being positioned in front of the objectivelens.

By an inaccurately conducting the positioning process, it is possiblethat collisions occur, which may result in damages to either the objector to components of the electron microscope.

It has been recognized, that the positioning of a sample inside of aparticle beam microscope is complicated to conduct. Thereby, handlingthe particle beam microscope for carrying out a positioning processwithin, a reasonable amount of time requires a lot of experience.

SUMMARY

Embodiments provide a method for operating a particle beam microscope,which comprises an objective lens having an object region, wherein themethod comprises: detecting light rays and/or particles, which emanatefrom a structure, wherein the structure comprises at least a portion ofa surface of an object and/or at least a portion of a surface of anobject holder of the particle beam microscope; generating a surfacemodel of the structure depending on the detected light rays and/orparticles; determining a position and an orientation of the surfacemodel of the structure relative to the object region; determining ameasurement location relative to the surface model of the structure; andpositioning the object depending on the generated surface model of thestructure, depending on the determined position and orientation of thesurface model of the structure, and depending on the determinedmeasurement location.

Accordingly, a method for operating a particle beam microscope isprovided, which allows to position a sample relative to a component of aparticle beam microscope, in particular an objective lens, with a highaccuracy. In particular, it is possible to position a location on theobject surface, at which a measurement is to be taken, in an objectregion of the objective lens with high accuracy and within a short time.Thereby, it is possible, even for an inexperienced user, to conduct ameasurement within a short time.

By way of example, the particle beam microscope may be a scanningelectron microscope. Further examples of a particle beam microscopes area focused ion beam systems, in particular helium ion microscopes.

The generating of the surface model of the structure is performeddepending on the detected light, rays and/or particles. The surfacemodel may be generated depending exclusively on the detected light rays.In other words, the surface model is generated exclusively from theinformation, which is obtained by the detected light rays.

However, it is also conceivable that additional information is used forgenerating the surface model. For example, the generating of the surfacemodel may be performed depending on values, which are obtained bymeasurements, which are carried out in addition to a detecting of thelight rays and/or particles. Thereby, it is possible to increase thespeed for generating the surface model. A surface model may for examplebe determined depending on a measurement conducted by a coordinatemeasuring device. Furthermore, a surface model of at least a portion ofthe structure, in particular of at least a portion of the surface of theobject holder, may be generated based on a CAD drawing.

The detecting of the light rays may be carried out with alight-sensitive sensor, in particular a semiconductor sensor. Thegenerating of the surface model may be carried out by a computer. Thepositioning of the object may comprise an automatic positioning, whichis controlled by the computer.

Furthermore, the detecting of the light rays may be performed by a lightsensitive image capturing device. The image capturing device maycomprise an image sensor, such as a CCD image sensor. A light sensitiveimage capturing device may for example comprise a camera, in particulara CCD-camera. The light sensitive image capturing device may beconfigured and arranged such that a digital image is acquirable, whereinthe digital image represents or shows at least a portion of thestructure. Furthermore, it is conceivable that the detected light raysare laser beams, which are scattered or reflected at the structure. Thelaser beams may be generated by a laser scanner which scans thestructure. Based on the detected laser beams, at least one of thefollowing may be performed: a time-of-flight measurement by timing theround-trip time of a pulse of light, phase comparison and/ortriangulation. The image sensor of the light sensitive image capturingdevice may for example comprise a CCD-image sensor and/or a photodiode.

The light rays may have, wavelengths within a range from 400 nanometersto 700 nanometers. The light rays may be emitted from a light source andmay be scattered or reflected at the structure. By way of example, inthe specimen chamber of the particle beam microscope, a light source maybe arranged, which illuminates the interior of the specimen chamber. Thelight rays may be light rays of a laser beam, which is emitted by alaser scanner, wherein the laser scanner is configured such that itscans the surface of the structure with the laser beam. Alternativelyadditionally, it is also conceivable that the light rays are emittedfrom light sources which are arranged at the structure. Such light,sources may for example be light-emitting diodes (LEDs).

The detecting of the light rays may be performed when the object and/orobject holder is in the specimen chamber. Alternatively or additionally,the detecting of the light rays may be performed when the object and/orobject holder is outside of the specimen chamber. For example, thedetecting of the light rays may be performed in a load-lock chamber ofthe particle beam microscope. The load-lock chamber may be configuredsuch that objects are first loaded into the load-lock chamber. After anevacuation of the load lock chamber, the objects are transferred intothe specimen chamber. Thereby, the specimen chamber does not have to beventilated for inserting new specimens. Thereby, the time in which theload-lock chamber is evacuated may be used to detect the light rays andto generate the surface model. It is also conceivable that the detectingof the light rays is performed outside of the vacuum system, whichcomprises the load-lock, chamber and the specimen chamber. For example,the detecting of the light rays may be performed under atmosphericpressure.

The detected particles may be charged particles. The particles may beelectrons. The electrons may be secondary electrons and/or backscattered electrons. Furthermore, the particles may be ions, such ashelium ions or secondary ions.

The particles emanate from the structure. The particles may be emittedfrom a portion of object, which is irradiated by the primary beam of theparticle bean microscope. In other words, the particles may be emittedfrom an impingement location or an impingement region of the primarybeam. The primary beam may be a scannable primary beam.

The detecting of the particles may be performed by one or more particledetectors. The particle detectors are configured such that particles aredetected, which are emitted from an impingement location of the particlebeam.

The object region may be defined as a spatial region relative to theparticle beam microscope, wherein the particle beam microscope isconfigured such that an image is acquirable from a portion of an object,which is arranged in this spatial region. In other words, the objectregion may represent a spatial region, which is scannable by the primarybeam of the particle beam microscope.

By way of example, the object is a wafer or a work piece. The scanningelectron microscope may be used to acquire an image of a surface of thewafer or the work piece.

The structure may be a surface. The surface may be three-dimensional.The structure may be a surface, which comprises at least a portion ofthe surface of the object and/or at least a portion of the surface ofthe object holder. The structure may consist of a surface, which ismovable relative to the object region by the positioning device. It isfurther conceivable that structure comprises at least a portion of asurface of a further component of the particle beam microscope. It isfurther conceivable that the structure does not comprise the total orthe total exposed surface of the object. The structure does not have tocomprise a surface of the object. For example, in case of objects beingrelatively small compared to a size of the object holder, it might besufficient, that the structure comprises a portion of the surface of theobject holder without any portion of the surface of the object. Theobject holder may be defined as a component of the particle beammicroscope, which is configured to retain an object on whichmeasurements are to be taken. By way of example, the object holder maycomprise a surface, at which the object is attached. The object, may beattached to the object holder by adhesive and/or by screws of the objectholder. The object may be attached to the object holder and the objectholder may be attached to the positioning device. The object holder maybe configured to provide a mechanical connection between the object andthe positioning device. In other words, the object and the object holdermay be positioned simultaneously within the particle beam microscope bythe positioning device.

The surface model may be a model, which represents the form or shape ofthe structure. In other words, the surface model of the structure may bea mathematical representation of the structure. For example, a maximumdistance of the surface model from the structure may be less than 10millimeters or less than 1 millimeter or less than 0 millimeter or lessthan 10 micrometers or less than 1 micrometer or less than 100nanometers or less than 10 nanometers. The distances may be measuredalong a surface normal of the surface model, wherein the surface modelis positioned relative to the structure such that the sum or integral ofthe squared distances yield a minimum.

Hence, the surface model may represent the structure to a predeterminedaccuracy. The accuracy of the surface model may be chosen such that apositioning of the structure in relative to the objective lens may becarried out with a predetermined positioning accuracy. For examplepositioning accuracy may be lower than 100 nanometers, lower than 1micrometer, lower than 10 micrometers, lower than 0.1 millimeter, lowerthan 0.5 millimeter, lower than 1 millimeter or lower than 5millimeters.

The surface model may represent a flat two-dimensional structure. Forexample, the surface model of a wafer may be a circular disc, whereinthe edge of the circular disc represents the outer edge of the wafer.The surface model may be a three-dimensional surface model. Athree-dimensional surface model may be defined such that it comprises anuneven surface. By way of example, a three dimensional surface model mayrepresent a lateral surface and a top surface of a cylinder or a cuboid(i.e. without its base).

By way of example, the surface model may comprise or consist of aplurality of points. In other words, the surface model may comprise ormay consist of a point cloud. The number of points may, for example, bemore than 10, more than 100, more than 1,000 or more than 10,000.Furthermore, the number of points may, for example, be less than 10¹⁰points or less than 10⁹ points. Each of the points may be defined bythree coordinate values, which represent a position of the points inspace relative to a coordinate system.

At least a portion of the points may be connected by geometric objectslike line segments, polygons, plane segments arcuate surface segmentsand/or arcuate line segments. The plane segments may comprise triangularand/or trapezoidal plane segments. For each point, the distances betweenthe point and its closest neighboring point may, be less than 5millimeters or less than 1 millimeter or less than 0.1 millimeter orless than 10 micrometers or less than 1 micrometer or less than 100nanometers or less than 10 nanometers.

Additionally or alternatively, the surface model may at least partly bebased on splines. In other words, the surface model may be based on aset of polynomial surface functions, wherein a polynomial surfacefunction describes at least a portion of the surface model. A pluralityof polynomial surface functions of a degree less or equal to four may besufficient for achieving a predetermined accuracy of the surface model.

The surface model may further comprise marks, wherein the markscorrespond to marks on the structure. For example, the structure maycomprise marks, which are detectable by the detecting of the light raysand/or particles. Such marks may, for example, be color coded marks orportions on the structure, which have a reflectivity, which is differentfrom a reflectivity of portions of the structure, which surround themarks.

The objective lens may, be an electron beam objective lens or anobjective lens for focused ion beams. Furthermore, also other componentsof the particle beam microscope may comprise object regions, such as aparticle detector or a component for object preparation. Examples forparticle detectors are secondary electron detectors (also denoted asSE-detectors), energy dispersive detectors for X-rays (also denoted asEDX detectors) and electron back scattered electron detectors (alsodenoted as EBSD detectors). Examples for components for objectpreparation are gas injection systems, focused ion beam systems (FIB)and micromanipulators.

Furthermore, the position and orientation of the surface model relativeto the object region is determined. The determining of the position andorientation of the surface model may comprise interpolating of points ofthe surface model.

A rigid body comprises six degrees of freedom of movement. The sixdegrees of freedom of movement are, for example, expressed by threecoordinate values of translation and three rotation angle values. Undertranslation, all points of the rigid body move by the same translationvector. The three coordinate values of translation together define theposition of the rigid body. Under rotation, all points of the rigid bodyare rotated by an angle about a rotation axis. The three rotation anglesdefine the orientation of the rigid body. The orientation of the surfacemodel may be expressed by yaw, pitch and roll or by Euherian angles.

The determining of the position and orientation of the surface modelrelative to the object region may be performed such that the positionand orientation of the surface model is aligned to a position andorientation of the structure relative to the object region.

The determining of the position and orientation of the surface model ofthe structure may be performed depending on the surface model of thestructure. For example, an extent of the structure and/or distancesbetween marks of the structure may be known from the determined surfacemodel of the structure. Furthermore, the determining of the position andorientation of the surface model relative to the object region may beperformed depending on the detected light rays. In particular, theposition and orientation may be determined depending on a digital imageof a light sensitive image capturing device, wherein the digital imagedepicts at least a portion of the structure. Additionally oralternatively, the determining of the position and orientation may beperformed depending on the signals, which are transmitted between thecomputer and the positioning device. For example, the positioning devicemay comprise a measuring unit, which is configured to measure theposition and/or orientation of the structure. Additionally oralternatively, the position and/or orientation of the structure may bedetermined depending on control, signals, which are transmitted from acontroller to the positioning device. The controller may, for example,be a computer. Additionally or alternatively, the determining of theposition and the orientation of the surface model of the structure maybe performed depending on detected particles, which emanate from thestructure. Particle detectors may detect the particles at differentfocus distances of the primary beam. Additionally or alternatively,determining of the position and orientation of the surface model of thestructure may be performed depending on particle microscopic images,which depict at least a portion of the structure.

The positioning of the structure may be performed by a positioningdevice of the particle beam microscope. The positioning device maycomprise one or more actuators. The object holder may be arranged at thepositioning device. Thereby, the positioning device may be configuredsuch that by controlling one or more actuators, the object ispositionable in the particle beam microscope relative to the objectivelens, relative to a detector and/or relative to a component for objectpreparation. The positioning may, in particular, comprise a positioningof the measurement location in the object region of the objective lens.Furthermore, the positioning may also comprise an adjusting of ameasurement orientation. The measurement orientation may be defined asan orientation of the object, at which a measurement is carried out. Ameasurement orientation, for example, may be defined by three angles ofrotation.

The measurement location may represent a portion on the surface of theobject, at which a measurement is to be taken or at which particlemicroscopic image is to be acquired. The measurement location may belocated outside of the surface model of the structure. The determiningof the measurement location relative to the surface model may beperformed depending on a user input via the computer. For example, theuser may select, a portion of the surface model in which he wants toperform a measurement or acquire an image, based on a two dimensionalrepresentation of the surface model on a display of the computer.Depending on the user input, the computer may determine or calculate ameasurement location relative to the surface model.

The positioning is performed depending on the determined surface model.The positioning may comprise interpolating points of the surface modelof the structure. Depending on the surface model and the measurementlocation relative to the surface model, a positioning direction may bedetermined for arranging the measurement location in the object region.Furthermore, based on the surface model, the user or the computer maydetermine in which measurement orientation the measurement is to betaken or the image is to be acquired. The positioning of the object maybe controlled by the computer. However, it is also conceivable that theuser manually controls the positioning of the object, wherein forexample the surface model of the structure, the position and orientationof the surface model of the structure and the measurement location isdisplayed on a display of the computer. Depending on the user input, thecomputer positions the object.

According to a further embodiment, the positioning of the object furthercomprises a determining of a positioning path. The positioning path maybe determined by a computer depending on the surface model, on thedetermined position and orientation of the surface model relative to theobject region, the measurement location and/or the measurementorientation. The positioning path may be determined such that themeasurement location is located in the object region. Furthermore, thepositioning path may be determined such that the positioning is carriedout without collision.

According to an embodiment, the positioning of the object comprisesarranging the measurement location in the object region.

According to a further embodiment, the method further comprisesadjusting of a focus of the objective lens after having arranged themeasurement location in the object region.

By arranging the measurement location in the object region according tothe method, the position and orientation of the structure relative tothe objective lens is known to a comparatively higher accuracy The focusof the scanning electron microscope is typically adjusted with anaccuracy, which is in a range between a few nanometers (nm) to a fewmicrometers (μm), depending on the set magnification of the scanningelectron microscope. The adjusting of the focus may be performedautomatically by setting operation parameters of the particle beamoptical system depending on acquired particle microscopic images. As aresult of the determining of the position and orientation of thestructure with high accuracy, an automatic adjustment of the focus isalleviated. Thereby, in particular, an adjusting of the focus may beperformed within a short time.

According to an embodiment, the method further comprises: generating asurface model of a microscope portion of the particle beam microscope;combining the surface model of the structure and the surface model ofthe microscope portion to generate a combined surface model; andcalculating a distance between the surface model of the structure andthe surface model of the microscope portion depending on the combinedsurface model; wherein the positioning of the object comprisesmonitoring the distance.

Accordingly, it is possible to quickly move the object within theparticle beam microscope without risking collisions, which may damagethe object or the particle beam microscope. In particular, a securepositioning is enabled for objects having a complex geometry or for aplurality of objects which are together mounted on the object holder.

The microscope portion may be at least a portion of a surface of acomponent of the particle beam microscope. Examples for such a componentare: the specimen chamber, a detector, a manipulator, a as supply and/oran objective lens.

The combined surface model may be defined as a surface model, in whichthe surface model of the structure and the surface model of themicroscope portion are arranged relative to each other corresponding tothe relative arrangement of the structure and the microscope portion inthe specimen chamber. The combining of the surface models may beperformed by the computer. The surface model of the microscope portionsmay comprise points and/or geometric objects, such as has been describedwith respect to the surface model of the structure.

The combining to generate the combined surface model may comprise:determining a position and an orientation of the surface model of thestructure relative to the surface model of the microscope portion.

The determining of the position and orientation of the surface model ofthe structure relative to the surface model of the microscope portionmay comprise acquiring a digital image, which represents or shows atleast a portion of the structure, wherein the digital image is acquiredfrom a viewpoint position relative to the microscope portion. Thedigital image may be generated by a light sensitive image capturingdevice, and/or the digital image may be a particle microscopic image.Additionally, the digital image may show at least a portion of themicroscope portion.

The acquired digital image may then be compared with the surface modelof the structure. Depending on the comparing, a position and orientationof the surface model of the structure relative to the surface model ofthe microscope portion may be determined. The comparing may comprisesegmenting of the digital image. The segmenting may comprise one or acombination of the following methods: a pixel-oriented method, anedge-oriented method, a region-oriented method, a model-based method, atexture-based method and/or a color-oriented, method. In particular, thecomparing may comprise a model-based segmentation method depending onthe surface model of the structure.

Additionally or alternatively, the method may comprise extractingfeatures from the digital image, wherein the extracted featurescorrespond to features of the surface model of the structure. Examplesof such features are: edges, surface topography, and/or detectablemarks. The comparing may comprise applying a routine for edge detection,for frequency filtering and/or for pattern recognition. Furthermore, thecomparing may comprise interpolating points of the surface model.

Additionally or alternatively, the combining to a combined surface modelmay be performed depending on signals, which are transmitted between thecomputer and the positioning device. For example, the positioning devicemay comprise a measuring unit, which is configured to determine theposition and orientation of the structure relative to the microscopeportion. Furthermore, the position and/or orientation of the surfacemodel of the structure relative to the surface model of the microscopeportion may be determined depending on control signals, which aretransmitted from a controller to the positioning device. The controllermay, for example, be the computer. Alternatively or additionally, thecombining to the combined surface model may be performed depending ondetected particles, which emanate from the structure. Particle detectorsmay detect particles at different focus distances of the primary beam.Alternatively or additionally, the determining of the position andorientation of the surface model of the structure relative to thesurface model of the microscope portion may be performed depending onparticle microscopic images, which represent or show at least a portionof the structure.

Depending on such a combined surface model, a distance between thestructure and the microscope portion is determinable. The detecting ofan imminent collision between the microscope portion and the structuremay be performed depending on the determined distance.

According to an embodiment, the method comprises determining of apositioning path depending on the combined surface model. Thepositioning path may be calculated by the computer.

The distance may represent a minimum distance between the structure andthe microscope portion. The minimum distance between two bodies may bedetermined by determining a smallest distance between any two points ofthe two bodies, wherein the line between the two points connects the twobodies.

For example, the determining of the distance may comprise comparingdistances between pairs of points, wherein each pair comprises a pointof the surface model of the microscope portion and a point of thesurface model of the structure. Depending on the comparing, a pair ofpoints may be determined, which has a smallest distance of all pairs ofpoints. The distance may be calculated by the computer. Furthermore, thedetermining of the distance may comprise interpolating points of thesurface model of the structure and/or interpolating points of thesurface model of the microscope portion.

The determining of a distance may comprise determining or calculatingdistances between pairs of points, wherein each of the pair of pointscomprises a point of the structure and a point of the microscopeportion; and determining a pair of points, which has the smallestdistance among all pairs of points.

Algorithms for determining collisions on the basis of surface models aredisclosed in the Ph.D thesis “Virtual Reality in AssemblySimulation—Collision Detection, Simulation Algorithms and InteractionTechniques” of Gabriel Zachmann (Technische Universitaet Darmstadt),published by Fraunhofer IPE Verlag; the contents of which areincorporated herein in its entirety. Furthermore, algorithms forcollision detection are disclosed in the article “SchnelleKollisionserkennung durch paraliele Abstandsberechnung” of DominikHenrich, et al., published in 13. Fachgespraech Autonome Mobile Systeme(AMS '97), Stuttgart, Oct. 6 and 7, 1997, published by Springer Verlag,series “Informatik Aktuell”; the contents of which are incorporatedherein in its entirety.

The monitoring of the distance may comprise issuing a notification or awarning signal by the particle beam microscope system, when the distancehas fallen, below a predetermined or predeterminable permissibledistance. Alternatively or additionally, it is conceivable that thepositioning of the object holder by the positioning device isautomatically stopped when the distance is smaller than the permissibledistance.

The permissible distance may be predetermined. The permissible distancemay be determined such that a collision between a structure and themicroscope portion is prevented. Furthermore, the permissible distancemay be determined taking into account an accuracy with which thestructure and the microscope portion are approximated by the combinedsurface model.

According to a further embodiment, the positioning of the objectcomprises determining of a positioning path depending on the combinedsurface model. The determining of the positioning path may comprise adetermining of distances between the surface model of the structure andthe surface model of the microscope portion along the positioning path.The positioning of the object may be performed depending on thedetermined positioning path.

By automatically determining the positioning path by the computer, afast and automatic positioning may be performed without collision.However, it is also conceivable that a user may perform a manualpositioning, wherein positioning movements which may lead to a collisionare prevented by notifications, warning signals, and/or a stopping ofthe positioning process.

According to a further embodiment, the determining of the position andorientation of the surface model of the structure relative to the objectregion comprises: generating a digital image, from at least a portion ofthe structure; and comparing the surface model of the structure with thedigital image.

The digital image may be acquired with a light-sensitive image capturingdevice. Alternatively or additionally, the digital image may be acquiredby scanning a portion of the structure with a primary beam of theparticle beam microscope. The digital image may be a particlemicroscopic image.

The comparing may comprise identifying features of the digital image,wherein the features of the digital image correspond to features of thesurface model of the structure or features of the combined surfacemodel. In other words, the comparing may comprise identifying featuresof the surface model, which are represented or shown in the digitalimage. Such features may, for example, comprise edges, marks and/orsurface topography of the structure and/or microscope portions. Thecomparing may comprise applying a routine for edge detection, forfrequency filtering and/or for pattern recognition. Furthermore, thecomparing may comprise interpolating points of the surface model. Thecomparing may comprise segmenting the digital image. The segmenting maycomprise one or a combination of the following methods: a pixel orientedmethod, an edge-oriented method, a region-oriented method, a model-basedmethod, a texture-based method. In particular, the comparing maycomprise a model-based method for segmentation depending on the surfacemodel of the structure.

The digital image may be compared with a two-dimensional representationof the surface model of the structure. The two-dimensionalrepresentation may be generated by projecting the surface model at agiven position and orientation onto a plane. The two-dimensionalrepresentation may be compared with the digital image to decide whetherthe given position and orientation corresponds to the position andorientation of the structure.

According to a further embodiment, the determining of the position andorientation of the surface model of the structure relative to the objectregion is performed depending on the digital image, depending on theviewpoint position of the image capturing device and depending on thesurface model of the structure.

According to a further embodiment, the method further comprises:determining a second measurement location relative to the surface modelof the structure and relative to the measurement location; andrepositioning the object depending on the measurement location and thesecond measurement location.

The repositioning may further be performed depending on the surfacemodel of the structure. The measurement location relative, to thesurface model of the structure may be stored, in particular in a storagedevice of the computer. The storing of the measurement location relativeto the surface model may comprise a storing of coordinates of a pointrelative to the surface model. Alternatively or additionally, ameasurement orientation relative to the surface model may be stored. Themeasurement orientation may be defined such that it represents theorientation of the structure when a measurement is taken.

The second measurement location may be the same measurement location asthe stored measurement location. Thereby, it is possible to find again alocation, at which a measurement has been taken.

Thereby, it is possible to readjust a measurement orientation and/or tofind a measurement location again after the object has been moved byoperating the positioning device. The object may have been moved, forexample, to perform a preparation outside of the particle beammicroscope. This allows to obtain measurements of exactly the samelocation and/or exactly the same orientation. Furthermore, it ispossible to assign stored images, which have been acquired with theparticle beam microscope to stored measurement locations and/ormeasurement orientations.

According to a further embodiment, the method further comprises:generating a particle microscopic image, which represents at least aportion of the measurement location; identifying a region of theparticle microscopic image; and adjusting a position and/or anorientation of the object depending on the identified region.

The adjusting in dependence on the identified region of the particlemicroscopic image may be performed at an accuracy, which is higher thanthe accuracy for the positioning in dependence of the surface model ofthe structure. In other words, the positioning in dependence of thesurface model of the structure may provide a coarse positioning, whichis followed by a fine positioning, which is performed in dependence onthe identified region of the particle microscopic image. In particular,it is possible to reproducibly find a measurement location again with anaccuracy, which corresponds to the resolution of the particlemicroscopic image.

The identifying of the region of the particle microscopic image maycomprise comparing the particle microscopic image with stored particlemicroscopic images. The stored particle microscopic images may have beenacquired during a preceding positioning process. Thereby, it is possibleto identify a portion of the object, where already a particlemicroscopic image has been acquired. Furthermore, the identifying of theregion of the particle microscopic image may comprise a segmenting ofthe particle microscopic image, an edge detection and/or a frequencyfiltering of the particle microscopic image. Thereby, features may bedetermined in the particle microscopic image, which are to be examinedby the particle be microscope. Based on the identified region of theparticle microscopic image, it is possible to determine a positioningpath for acquiring an image of the identified region at a highermagnification. The computer may be configured to perform the positioningdepending on lee identified region.

According to an embodiment, the detecting of the at light rays and/orparticles comprises detecting the light rays and/or particles at aplurality of different focus distances.

The focus distances may be focus distances of a light sensitive imagecapturing device and/or focus distances of the primary beam.

The focus distance of the primary beam may be a distance of a beam waistof the primary beam of the particle microscope from a reference point ofthe particle optical system of the particle beam microscope. Thereference point may, for example, be a principal, plane of the objectivelens or a component of the particle optical system of the particle beammicroscope. The focus distance of the light sensitive image capturingdevice may be a focus distance of a light optical system of the lightsensitive image capturing device, such as a lens assembly.

According to an embodiment, the generating of the surface model of thestructure further comprises: generating a plurality of stacks of imageregions depending on the detected light rays and/or the detectedparticles at the plurality of focus distances; wherein image regions,which are cart of a same stack of the plurality of stacks represent asame portion of the structure; determining for each stack of theplurality of stacks an in-focus region depending on the image regions ofthe respective stack.

Each of the image regions may be a group of pixels of the digital image.The digital image may be acquired at a focus distance of the lightsensitive image capturing device and/or of the primary beam. Each of theimage regions may be generated by selecting pixels from the digitalimage. All pixels of an image region may be generated at the same focusdistance.

Image regions, which form part of a same stack, show a same portion ofthe structure. Image regions, which form part of a different stack mayshow different portions of the structure. The different portions of thestructure may be adjacent. The adjacent portions may be non-overlapping.Alternatively, the different portions may partly overlap each other.Furthermore, the different portions may be spaced apart from each other.

The in-focus region is determined by determining the image region fromall image regions of a stack, which has the highest resolution. Thedetermining of the in-focus region may comprise comparing all imageregions of a stack. The determining of the in-focus region may comprisedetermining frequencies, in particular spatial frequencies, of imagedata values for each image region of a stack. The frequencies may befrequencies of a row and/or a column of the image region. For example,the determining of a frequency may comprise determining a Fouriertransform, in particular a discrete Fourier transform a of at least aportion of the image data of an image region. For example, the imageregion, which has a highest frequency in its power spectrum is thein-focus region. Furthermore, the in-focus region may be the imageregion having the greatest power values in the power spectrum at apredetermined frequency or within a predetermined frequency range.Additionally or alternatively, the determining of the in-focus regionmay comprise determining of differences and/or gradients of image datavalues of the image regions of the stack. For example, the image regionhaving the highest absolute values of differences of neighboring imagedata values is determined, as the in-focus region. Additionally oralternatively, determining of the in-focus region may comprise applyingan edge detection filter to each image region of a stack.

The determining of the in-focus region may be performed depending onpixel data values of the image regions of the respective stack.Alternatively or additionally, the determining of the in-focus regionmay be performed depending on pixels outside of the image region. Forexample, the determining of the in-focus region may be performeddepending on pixels, which are adjacent to or spaced apart from thepixels of the image region of the respective stack. Thereby, it is inparticular possible that an image region consists of a single pixel.

According to a further embodiment, each image region, of at least aportion of the generated image regions is an isolated pixel cluster.

A pixel cluster may be defined as a group of pixels, wherein each of thepixels is located adjacent (i.e. not spaced apart) to at least one otherpixel of the pixel cluster. An isolated pixel cluster may be defined asa pixel cluster, wherein each pixel of the isolated, pixel clusterspaced apart from a pixel, of another image region of a different stack.In other words, the portion of the structure, which is represented orshown by the pixel cluster is neither adjacent nor overlapping, butspaced apart from portions of the structure, which are represented byother image regions which form part of a different stack.

Each of the isolated pixel clusters may consist of between 1 and 8pixels, between 1 and 50 pixels or between 1 and 500 pixels, or between1 and 1,000 pixels, or between 1 and 10,000 pixels. In particular, apixel cluster may consist of an individual pixel.

A minimum distance between a first and a second pixel cluster may bedefined as a smallest distance of all distances between pixels of thefirst pixel cluster and pixels of the second pixel cluster.

The minimum distance between pixel clusters of different stacks may bemore than 10 times, more than 100 times or more than 1,000 times thediameter of the pixel. In other words, a distance between regions of thestructure, which are represented by isolated pixel, clusters ofdifferent stacks may be many times more than a sampling distance betweenpixels of the image region. A sampling distance may be defined as adiameter of a portion of the structure, which is represented by a pixel.

The acquiring of the image data of the image region may comprise ascanning with the primary beam structure regions, which connect theisolated pixel clusters. The isolated pixel clusters may then be cut outfrom the acquired image. Thereby, it is possible, that only a smallnumber of pixel data values have to be processed by the computer togenerate the surface model of the structure.

Alternatively, the generating of the image regions may comprise skippinga scanning of structure portions, which connect the isolated pixelclusters. In other words, the structure portions, which connect theisolated pixel clusters are not scanned by the primary beam. This allowsto generate a surface model having a comparatively large structurewithin a short time.

According to a further embodiment, the method further comprises:generating digital image data, which represent at least a portion of thestructure depending on the detected light rays and/or the detectedparticles; wherein the generating of the surface model of the structureis performed depending on the digital image data.

The digital image data may be pixel data values of a group of pixels, inparticular of a digital image. The pixel data values may represent colorand/or gray scale values. The digital image data values may represent atleast a portion of the structure. The digital image data may be acquiredby a light-sensitive image acquisition device and/or by a scanning ofthe primary beam.

Based on the digital image data, the surface model may be calculated bya computer. Such algorithms are, for example, described in the article“3D Reconstruction from Multiple images: Part 1 Principles” of TheoMoons, Luc van. Cool, and Maarten Vergauwen, published in “Foundationsand Trends in Computer Graphics and Vision”, Volume 4, Issue 4, pages287 to 404; the contents of which are incorporated herein in itsentirety. Furthermore, such algorithms are described in the article“DLP-Based 3D Metrology by Structured Light or projected FringeTechnology for Life Sciences and Industrial Metrology” of G. Frahkowskiand R. Hainch, published, in “Proceedings of SPIE Photonics West 2009”;the contents of which are incorporated herein in its entirety.Furthermore, such algorithms are described in the article “ProFORMA:Probabilistic Feature-based On-line Rapid Model Acquisition.” of Qi Panet al., published in the Proceedings of the “BMVC 2009” of the BritishMachine Vision Association, London (obtainable on the webpagehttp://www.bmva.org/bmvc/2009/index.htm), the contents of which areincorporated herein in its entirety.

Alternatively or additionally, it is conceivable that, for example,based on further measurements at the structure, a course model isavailable which is adapted depending on the digital image data. Forexample, a surface model of at least a portion of the surface of theobject holder may be stored in the storage device. Depending on thedigital image data, the stored surface model of the portion of thesurface of the object holder is supplemented to yield a surface model ofthe structure.

Thereby, the surface model of the structure may be obtained from thedigital image data within a short time.

The position and orientation of the surface model relative to the objectregion or relative to the surface model of the microscope portion may bedetermined depending on the digital image data. By way of example, aviewpoint position relative to the object region from which the digitalimage data are acquired, an imaging direction and/or a magnification ofthe digital image data may be known. Thereby, it is possible todetermine the position and orientation of the surface model.

The acquiring of the digital image of the structure may be performed bya light-sensitive image capturing device, such as for example a camera.

According to a further embodiment, the generating of the surface modeldepending on the digital image data comprises a segmenting of thedigital image data. The segmenting may further comprise one or acombination of the following segmenting methods: a pixel-orientedmethod, an edge-oriented method, a region-oriented method, a model-basedmethod and a texture-oriented method. An example of a pixel-orientedmethod is the threshold method. Examples for edge-oriented methods are:applying the Sobel-Operator, applying the Laplace-Operator and/orgradient detection. Examples for a region-oriented method are: RegionGrowing, Region-Splitting, Pyramid Linking and Split and Merge. Anexample for a model-based method is the Hough-Transformation. Examplesfor a texture-based method are co-occurrence Matrices andTexture-Energy-Measure.

According to a further embodiment, the generating of the digital imagedata comprises generating the digital image data from at least twodifferent imaging directions.

The image data, which have been acquired from the at least two differentimaging directions may represent stereoscopic image data. For example ortwo or more images are acquired from different imaging directionsrelative to the structure. Depending on the stereoscopic image data, thesurface model of the structure, the position and/or orientation of thesurface model of the structure relative to the object region, and/or theposition and orientation of the surface model of the structure relativeto the surface model of the microscope portion may be determined.

The acquiring of the digital images from different imaging directionsmay, for example, comprise varying of the orientation and/or position ofthe structure relative to a light-sensitive image capturing deviceand/or relative to the primary beam. For example, the orientation and/orthe position of the structure may be varied by the positioning device.Thereby, the structure may be imaged by the camera or the primary beamfrom different imaging directions. An imaging direction may be definedby a vector, which is parallel to the optical axis of thelight-sensitive image capturing device or parallel to the optical axisof the particle optical system.

Additionally or alternatively, the imaging direction may be altered byvarying an impingement direction of the primary beam relative to anoptical axis of the particle beam microscope. Additionally oralternatively, a variation of the position of the light sensitive imagecapturing device relative to the specimen chamber may result in avariation of the imaging direction of the light sensitive imagecapturing device.

Additionally or alternatively, the light sensitive image capturingdevice may have more than one imaging direction. For example, thelight-sensitive image capturing device may comprise, a plurality ofcameras, which are arranged such that they have different imagingdirections relative to the structure. For example, the image capturingdevice comprises two, three or more cameras.

Additionally or alternatively, the particle optical system may provide afirst imaging direction and the light sensitive image capturing devicemay provide a second imaging direction.

According to a further embodiment, the detecting of the light, rayscomprises: detecting of a laser beam, which has been reflected at thestructure.

Algorithms for generating surface models from reflected laser beams of alaser scanner are disclosed in the Ph.D thesis “Model-based Analysis andEvaluation of Point Sets from Optical 3D Laser Scanners”, written byChristian Teutsch (Otto-von-Guericke-Universitaet, Magdeburg, Germany),published by Shaker Verlag, Herzogenrath, Germany; the contents of whichare incorporated herein in its entirety.

For example, the particle beam microscope comprises a laser scanner,which is configured to scan at least a portion of the structure and/orthe microscope portion. The laser scanner may be configured such thatthe reflected laser beams are detected by performing at least one of thefollowing: measuring the time-of-flight, in particular by timing theround-trip time of a pulse of light, performing phase comparison and/orperforming triangulation.

Furthermore, the laser scanner may be configured to determine theposition and orientation of the structure depending on the detectedreflected laser beams. Thereby, a position and an orientation of thesurface model of the structure relative to the object region may bedetermined.

According to a further embodiment, the generating of the surface modelof the structure comprises: generating a first surface model of a firstportion of the structure in a first position of the structure relativeto an image acquisition device and/or to the objective lens; generatinga second surface model of a second portion of the structure in a secondposition of the structure relative to the image acquisition deviceand/or the objective lens; and combining the first surface model and thesecond surface model to the surface model of the structure.

Accordingly, it is possible to generate a comparatively large surfacemodel, which extends the field of view of the image acquisition deviceor the particle microscope. In particular, this allows to use theparticle microscope to generate a surface model of an extended object.

The first and the second surface model may be generated depending on thedetected light rays and/or particles. The first surface model and thesecond surface model may be adjacent and non-overlapping. Alternatively,the first surface model, and the second surface model may be partiallyoverlapping. The first position and the second position are measuredrelative to the image acquisition device and/or relative to theobjective lens.

Embodiments provide a particle beam microscope system, comprising: anobjective lens, having an object region; an object holder which isconfigured such that an object is mountable on the object holder; apositioning device, which is configured to adjust a position and/or anorientation of the object holder relative to the object region; adetecting device, which is configured to detect light rays, whichemanate from a structure, and/or particles, which emanate from thestructure, wherein the structure comprises at least a portion of thesurface of the object holder and/or at least a portion of a surface ofthe object; a computer, which is configured for signal communicationwith the positioning device and the detecting device, wherein thecomputer is further configured to: generate a surface model of thestructure depending on the detected light rays and/or the detectedparticles; determine a position and an orientation of the surface modelof the structure relative to the object region; determine a measurementlocation relative to surface model of the structure; and to position theobject depending on the determined surface model of the structure, thedetermined position and orientation of the surface model of thestructure and the determined measurement location.

Accordingly, a article beam microscope is obtained, which allows anautomatic, fast and easy-to-perform positioning of the object relativeto the objective lens within a short time.

The computer may be configured to automatically perform the positioningof the object. It is also conceivable that the computer displays thesurface model of the structure, the position and orientation of thesurface model of the structure and the measurement location on adisplay. The computer may further be configured to position the objectdepending on the user input. For example, the computer may be configuredto determine a measurement location relative to the surface model of thestructure depending on an input of the user.

BRIEF DESCRIPTION OF THE DRAWINGS

The forgoing as well as other advantageous features of the inventionwill be more apparent from the following detailed description ofexemplary embodiments of the invention with reference to theaccompanying drawings. It is noted that not all possible embodiments ofthe present invention necessary exhibit each and every, or any, of theadvantages identified herein.

FIG. 1 schematically illustrates an object and an object holder whichare arranged close to an objective lens and a BSE-detector according toan exemplary embodiment;

FIG. 2 schematically shows a particle beam system according to anexemplary embodiment;

FIG. 3 schematically shows a surface model of a structure which isobtained by a method according to an exemplary embodiment;

FIG. 4 schematically shows a combined surface model, which is obtainedaccording to an exemplary embodiment;

FIG. 5 schematically shows a determining of the position and theorientation of the surface model according to an exemplary method;

FIG. 6 is a flow-chart, which schematically illustrates an exemplarymethod of operating a particle beam microscope;

FIG. 7 is a flow-chart, which schematically illustrates a furtherexemplary method of operating a particle beam microscope;

FIG. 8 schematically illustrates the acquiring of the surface model of astructure by using different focus distances of the particle opticalsystem, as shown in FIG. 2;

FIG. 9 schematically illustrates the generating of the surface model ofthe structure from a plurality of particle microscopic images;

FIG. 10 schematically shows the surface model of the structure which hasbeen generated according to an exemplary method, as shown in FIGS. 8 and9;

FIGS. 11 a and 11 b schematically show the generating of a surface modelof a structure in the exemplary method as shown in FIGS. 8 and 9,wherein the structure is greater than a field of view of the particleLearn microscope; and

FIG. 12 schematically illustrates the generating of a surface model of astructure from particle microscopic images according to an exemplaryembodiment.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

It should be noted in this context that the terms “comprise”, “include”,“having” and “with”, as well as grammatical modifications thereof usedin this specification or in the claims, indicate the presence oftechnical features such as stated components, figures, integers, stepsor the like, and by no means preclude the presence or addition of one ormore alternative features, particularly other components, figures,integers, steps or groups thereof.

FIG. 1 schematically shows a structure, which is arranged close to anobjective lens 30 of a particle beam microscope, which is, for example,a scanning electron microscope. The objective lens 30 has an opticalaxis OA and an object region OR. The object region OR is a spatialregion, into which the particle beam of the particle beam microscope isfocused. In other words, a surface region of an object, which isarranged in the object region OR, is imagable by the particle beammicroscope. The object region OR is located at a working distance WDspaced away from the objective lens 30. The working distance WD and theextent of the object region OR depend on the design of the particleoptical system of the particle beam microscope as well as on operationparameters of the particle optical system, such as the magnification.

A first object 10 and a second object 11 and a third object 12 aremounted on an object holder 20. The object holder 20 is attached to apositioning device, which is not illustrated in FIG. 1. The positioningdevice is configured such that the object holder 20 is independentlymovable along an X-axis, along a Y-axis and a Z-axis of a coordinatesystem. This is illustrated by the double arrows 50, 51 and 53. Thereby,the positioning device is configured to position the object holder withthree degrees of freedom. The positioning device may further beconfigured such that the object holder 20 is rotatable about the X-axis,the Y-axis and the Z-axis. In FIG. 1 this is illustrated by arrows 54,55 and 56. Thereby, the positioning device may be configured such thatthe object holder 20 is positionable in six degrees of freedom. Thepositioning device may comprise one or more actuators. The actuators maybe piezo actuators and/or step motors.

At an end face of the objective lens 30, a detector 40 is arranged,which is configured to detect back scattered particles, which have beenscattered at the object 10. In case of the particle beam microscopebeing a scanning electron microscope, the detector 40 may be aBSE-detector (back scattered electron detector). The particle beammicroscope may comprise further particle detectors, which are notillustrated in FIG. 1

In order to acquire an electron microscopic image of a location M on thesurface of the first object 10, the first object 10 has to be arrangedat a position and an orientation, such that the location M is located inthe object region OR. The orientation may, for example, be defined bythree angles.

The object holder 20 may comprise marks 21, 22. The marks 21, 22 areconfigured such that they are detectable in an image of alight-sensitive image capturing device, such as a CCD-camera, and/or byscanning the primary beam of the particle beam microscope across themarks.

In the exemplary embodiments, which are discussed with reference to thefollowing figures, a surface model is generated from the structure forperforming a precise positioning of the objects 10, 11, 12 relative tothe objective lens 30. The structure comprises a portion of the surfaceof the objects 10, 11, 12 and/or a portion of the surface of the objectholder 20. Additionally or alternatively, also a surface model of amicroscope portion (such as a portion of the objective lens 30 and/orthe detector 40) is generated to ensure a collision-free positioning ofthe objects 10, 11, 12.

The surface models may be generated, for example from camera images,which are arranged in the specimen chamber and/or in the load-lockchamber of the microscope. The surface models may also be generated fromparticle microscopic images and/or by using a laser scanner.

FIG. 2 is a schematical illustration of a particle beam microscopesystem 1 according to an exemplary embodiment. The particle beammicroscopy system 1 may comprise a scanning electron microscope. Thespecimen chamber 80 comprises a vacuum pumping system 83, which isconfigured to evacuate the specimen chamber 80 to a vacuum level, whichis suitable for conducting measurements with the primary beam. Thevacuum pumping system 83 may comprise a fore pump and a turbo molecularpump. The vacuum level for conducting measurements may be in a range ofbetween 1 mbar to 10⁻⁷ mbar. In order to avoid venting the specimenchamber 80 for changing the samples 10, 11, 12, a load-lock chamber 85may be connected to the specimen chamber 80 which comprises a furthervacuum pumping system 81. The samples 10, 11, 12, which are attached tothe object holder 20, are first introduced into the load-lock chamber85. After having evacuated the load-lock chamber 85, the samples 10, 11,12 and the object holder 20 are transferred from the load-lock chamber85 to the specimen chamber 80 and the object holder 20 is attached tothe positioning device 60 of the particle beam microscope.

The particle beam microscope comprises a first camera 31, such as aODD-camera, which is arranged in the specimen chamber 80. The firstcamera 31 is configured to acquire digital images of at least a portionof the surface of the first object 10 and/or a portion of the surface ofthe object holder 20. The first camera 31 is connected to the computer70 of the particle beam microscope system 1 via a first signal line 34.The computer 70 comprises a storage device 71. The storage device 71 isconfigured to store the digital image of the first camera 31. Thepositioning device 60 may be configured such that the first, second andthird object 10, 11, 12 and the object holder 20 are imagable by thefirst camera 31 from different imaging directions. For example, thepositioning device 60 may perform a rotation about the Z-axis by apredetermined angle, such that the first, second and third object 10,11, 12 and/or the object holder 20 is imagable by the first camera 31from at least two different imaging directions. Depending on the imagesof the first camera 31, the computer 70 calculates a surface model ofthe structure, which comprises at least a portion of a surface of thefirst, second, third object 10, 11, 12, and/or the object holder 20.

The particle beam microscope 1 may further comprise a second camera 32,such as a COD-camera, which is also arranged in the specimen chamber 80.The second camera 32 and the first camera 31 have different imagingdirections relative to the structure. By using two cameras, it ispossible to acquire digital images from the structure from differentimaging directions, without having to change the position or orientationof the structure of the positioning device 60.

The particle beam microscope system 1 further comprises a particleoptical system 39, which has an objective lens 30. The objective lens 30comprises an end face, which faces the object plane of the particleoptical system 39. At the end face, a detector 40, such as aBSE-detector may be arranged. It is also conceivable that the detectoris attached to a wall of the specimen chamber 80 or is received withinthe particle optical system. The particle optical system 39 and thedetector 40 are connected to the computer 70 via a third signal line 37.Through the third signal line 37, control signals are transmittedbetween the computer 70 and the particle optical system 39. Depending onthe signals of the detector 40, the computer 70 generates particlemicroscopic images, which represent digital images.

Digital images, which have been acquired by the first camera 31, and/orthe second camera 32 and/or which have been generated depending on thesignals of the detector 40 are stored in the storage device and laterprocessed by the computer 70. Depending on the digital images, thecomputer calculates a surface model of the structure. The structure canbe used to position the objects 10, 11, 12 relative to the objectivelens to acquire particle microscopic images.

The computer 70 is further configured to calculate a surface model of amicroscope portion of the particle beam microscope system 1 depending onthe digital images. Alternatively, it is possible that the computercalculates the surface model of the microscope portion depending on aCAD-model. The microscope portion may, for example, be a surface of anobject-side end portion of the objective lens 30 and/or a portion of thesurface of the detector 40. The computer 70 is further configured tocombine the surface model of the structure and the surface model of themicroscope portion to a combined surface model. The combined surfacemodel can be used to monitor a distance between the structure and themicroscope portion in order to avoid collisions during the positioningprocess.

A third camera 33, such as a CCD-camera, may be arranged in theload-lock chamber 85. The third camera is connected to the computer 70via a fourth signal line 36. Furthermore, the load-lock chamber 85 maycomprise a positioning device, which is configured such that digitalimages are acquirable by the third camera 33 from different imagingdirection relative to the structure. In the load-lock chamber 85, morethan one camera may be arranged. The cameras in the load lock chambermay be arranged such that they have different imaging directionsrelative to the structure.

The cameras in the load-lock chamber 85 may be configured to generatedigital image data, which show or represent at least a portion of thestructure, such that the surface model of the structure is calculabledepending on the digital image data. In the load lock chamber, the fieldof view of the camera is not obstructed by the presence of an objectivelens and/or detectors.

Depending on the generated surface model, the position and orientationof the structure in the specimen chamber 80 may be determined bycomparing the surface model with the digital images, which have beengenerated in the specimen chamber.

FIG. 3 schematically shows the generated surface model 90 of thestructure in the example, shown in FIG. 3, the structure comprises thetop surfaces and the lateral surfaces of the first, second and thirdobject 10, 11, 12. Furthermore, the structure comprises the top surfaceof the object holder 20. Those surfaces of the object holder, which arenot represented by the surface model of the structure 90 are indicatedin FIG. 3 by dashed lines. The surface model of the structure 90comprises a plurality of points 91, wherein the plurality of points 91are connected by geometric objects such as line segments or planesegments 91A.

Furthermore, the surface model of the structure 90 comprises marks 97,98, which represent the marks 21, 22 on the structure, as illustrated inFIG. 1.

After having generated lee surface model of the structure 90, thecomputer 70 (illustrated in FIG. 2) is configured to determine theposition and orientation of the surface model 90 relative to the objectregion OR, as will be discussed in detail with reference to FIG. 4.

The computer 70 is further configured to show a two-dimensionalrepresentation 73 on a display 72 of the computer 70, such asillustrated in FIG. 2. This allows the user to select a location, atwhich he wants to perform a measurement. The user may select a view ofthe representation 73 on the display. Based on the selected view, it iseasier for the user to decide at which location he wants to perform themeasurement. The representation 73 may be superimposed on a camera imageshowing the structure and/or the microscope portion.

Based the user input, the computer 70 determines a measurement locationP relative to the surface model 90. The measurement point P correspondsto a location M (as shown in FIG. 1), at which a measurement is to betaken.

Depending on the determined position and orientation, of the surfacemodel 90 relative to the object region OR, as well as depending on themeasurement location P, the computer calculates a positioning path P.

The positioning path may comprise translational movements and/orrotational movements. In FIG. 4, the positioning path T is schematicallyindicated as a vector, which connects the measurement location P withthe object region. OR. However, it is also conceivable that thepositioning path T comprises an arcuate path of translatory movement.After having determined the positioning path T, the computer transmitscontrol signals to the positioning device 60 for arranging a location onthe structure, which corresponds to the measurement location P withinthe object region OR.

FIG. 4 shows, in an exemplary manner, a combined surface model 93, whichhas been generated by combining the surface model of the structure 90with a surface model of a microscope portion 92. In this context, theterm combining may be understood to arrange the surface model of thestructure 90 and the surface model of the microscope portion 92 relativeto each other such that they represent the position and orientation ofthe structure relative to the microscope portion in the particle beammicroscope.

The surface model of the microscope portion 92 may be generateddepending on the detected light rays. Alternatively or additionally, thesurface model of the microscope portion 92 may be determined dependingon a contact-based measurement. The contact-based measurement may beperformed by a coordinate measuring machine.

The computer 70 is configured to calculate a distance D between thesurface model of the structure 90 and the surface model of themicroscope portion 92 depending on the combined surface model 93. Forexample, the computer calculates all distances between pairs of pointsof the combined surface models 93, wherein each pair of points consistsof a point of the surface model of the structure 90 and a point of thesurface model of the microscope portion 92. Depending on the determineddistances of the pairs of points, the smallest distance D may bedetermined. The distance D, which is shown in FIG. 4, is the distancebetween the point Q of the surface model of the microscope portion 92and the point R of the surface model of the structure 90. In case thedistance D is smaller than a predetermined permissible distance, theparticle beam microscope issues a warning signal or a notification.Furthermore, the particle beam microscopy system 1 may be configured tostop positioning movements which lead to a distance between themicroscope portion and the structure, which is smaller than thepermissible distance. The particle beam microscope system 1 isconfigured to determine a positioning path T depending on the combinedsurface model 93, wherein the positioning path T is determined such thata collision between the microscope portion and the structure is avoided.

FIG. 5 schematically snows in an exemplary manner the determining of theposition and orientation of the surface model of the structure relativeto the object region. After the surface model 90 has been generated, thefirst camera 31 (illustrated in FIG. 2) acquires a digital image 94, asillustrated an FIG. 5. In other words, the first camera 31 is a positionacquisition camera of the particle beam microscope system 1. Thecomputer 70 is configured to compare the digital image 94 with thesurface model of the structure 90. For example, the digital image 94 iscompared with two-dimensional representations 90A, 90B which representthe surface model in different orientations and positions. The comparingmay, for example, comprise extracting an edge 96 of the structure 90from the digital image 94 and comparing the extracted edge 96 with anedge or a rim 96A of the representation 90A of the surface model 90.Furthermore, the comparing may comprise extracting the mark 99, shown ina digital image 94, with a mark 99A of the representation 90A of thesurface model 90. The extracting of the edge 96A and/or the mark 99 maycomprise segmenting the digital image 94.

Based on the comparison, the two dimensional representation 90A isidentified as representing the position and orientation of thestructure. Thereby, the position and orientation of the surface model ofthe structure 90 is determined.

It is conceivable that the determining of the position and theorientation of the surface model 90 comprises determining digital imagesfrom at least two different imaging directions relative to thestructure. The digital images may represent stereoscopic image data.

FIG. 6 is a flow-chart of an exemplary method for positioning the objectwithin a particle beam microscope system 1, as shown in FIG. 2, by usingthe surface model of the structure 90, as shown in FIG. 3. A detecting100 of light rays, which emanate from the structure is performed by thefirst and/or second camera 31, 32. Depending on the geometry of thestructure and/or the required accuracy for the calculation of thesurface model 90, one or more images or the first camera 31 may besufficient to calculate the surface model of the structure 90. Theacquired digital images, which represent digital image data, aretransmitted via the first and the second signal lines 34, 35 to thecomputer 70 and are stored in the storage device 71. Depending on theacquired digital images, a generating 101 of the surface model 90 isperformed by the computer 70. The generated surface model 90 is storedin the storage device 71 of the computer 70.

Alternatively or additionally, the computer 70 may be configured tocalculate the surface model depending on signals of a particle detector,such as the detector 40, which is illustrated in FIG. 2. Exemplaryembodiments for calculating the surface model of the structure from thedetected particles will be discussed with reference to FIGS. 9 to 12.

Depending on the known viewpoint positions, the known imaging directionsand the known magnifications of the first and/or second camera 31, 32,and/or depending on the generated surface model 90, a determining 102 ofa position and orientation of the surface model of the structure 90relative to the object region OR is performed.

Alternatively or additionally, the determining 102 of the position andorientation of the surface model of the structure relative to the objectregion may be performed depending on signals between the positioningdevice 60 and the computer 70.

Alternatively or additionally, the determining 102 of the position andorientation of the surface model of the structure 90 is performeddepending on signals of a particle detector, such as the particledetector 40, as shown in FIG. 2. In particular, the determining 102 ofthe position and orientation of the surface model of the structure maybe performed depending on particle microscopic images.

The computer 70 is configured to display a two-dimensionalrepresentation 73 of the surface model on the display 72. Based on theshown representation 73, the user can select a location at which hewants to acquire a particle microscopic image. Depending on the userinput, the computer performs a determining 103 of a measurement locationP relative to the surface model of the structure 90.

Depending on the position and orientation of the surface model 90relative to the object region. OR and the determined measurement pointP, the computer determines 104 a positioning path. Depending on thedetermined positioning path T the computer transmits signals to thepositioning device 60 to control a positioning 105 of the object. Afterthe positioning of the object, the location of the object 10, at which ameasurement is to be taken, is arranged in the object region OR. Then,the computer 70 may again determine 102 the position and orientation ofthe surface model 90 or may determine 103 a measurement locationdepending on an input of the user.

FIG. 7 illustrates a flow-chart of a further exemplary method, which isperformed by the particle beam microscope system 1, as shown in FIG. 2,wherein the combined surface model 93, as shown in FIG. 4 is used forcollision detection. The method steps of detecting 110 the light raysand/or particles and generating 111 the surface model of the structureare performed as has been discussed with reference to FIG. 6.

In the exemplary method shown in FIG. 7, the computer generates 112 alsoa surface model of a microscope portion 92. The microscope portion 92may for example comprise at least a portion of the surface of thedetector 40, an objective lens 30, a manipulator, a gas injectionsystem, and/or a wall of the specimen chamber 80. Then, the computer 70combines the surface model of the structure 90 with the surface model ofthe microscope portion 92 to form a combined surface model 93. In thecombined surface model 93, the surface model of the structure 90 isarranged relative to the surface model of the microscope portion 92 suchthat it corresponds to a relative orientation and a relative position ofthe structure relative to the microscope portion in specimen chamber 80.The combining 113 may be performed depending on digital images of thefirst camera, the second camera 32, and/or signals of the detector 40.Alternatively or additionally, the combining 113 may be performeddepending on control and/or sensor signals between the positioningdevice 60 and the computer 70.

The surface model of the structure 90 and the surface model of themicroscope portion 92 may be generated consecutively. However, it isalso conceivable that the surface model of the structure 90 and thesurface model of the microscope portion 92 are generated simultaneously,in particular depending on the same digital images. Depending on thecombined surface model 93, a distance between the surface model of thestructure and the surface model of the microscope portion is determined.Depending on the combined surface model 93 and the determined distance,the computer 70 determines 115 a positioning path T. The positioningpath T is determined such that a collision between the structure and themicroscope portion is avoided. After the positioning 116, the computer70 again generates a combined surface model 93. After having againdetermined the distance, the positioning path is again determined suchthat the collision between the structure and the microscope portion isavoided. Then, the computer again controls the positioning 116 along thepositioning path T.

FIG. 8 shows in an exemplary manner how the surface model of thestructure is generated depending on image data, which have been acquiredby detecting particles. The image data are generated at different focusdistances of a primary beam 201 of the particle optical system 303(illustrated in FIG. 2). The primary beam 201 is scanned across thestructure 203. The primary beam 201 comprises a beam waist W. The beamwaist W is a portion of the primary beam 201, in which the primary beamhas a smallest, beam diameter measured perpendicular to a beam axis BAof the particle optical system. A region B of the structure 203, whichis located at a distance A away from the beam waist W is irradiated witha beam diameter of the primary beam 201, which is greater than the beamdiameter of the beam waist W.

During the scanning of the primary beam 201 across the structure 203,image data are generated. The image data represent a discrete samplingof the structure 203. For example, the image data may comprise 1024times 1024 pixel data values. Therefore, each pixel data valuerepresents a portion of the structure 203, having a diameter D. Forexample, M times M pixel data values are acquired from a square-shapedportion of the structure having side lengths L. The diameter of theportion of the structure 203, which is represented by a pixel, datavalue is L/M.

In case the diameter of the primary beam at the irradiated portion B isgreater than the diameter D, this causes a lower resolution in the imagedata of the digital image. A depth of focus T of the primary beam 201may be defined as a range along the beam axis BA, in which the diameterof the particle beam 201 is smaller than the diameter D. The depth offocus T depends on an aperture angle α of the primary beam 201. Theaperture angle α may be defined as a maximum angle, which is formed bythe particles of the primary beam 201 with the beam axis BA.

When the distance A of the portion B of the object surface OS from thebeam waist W is smaller or equal to half of the depth of focus T, thisdoes not cause a reduced resolution in the image data of the digitalimage. However, in case the distance A is greater than half of the depthof focus T, this leads to a reduced resolution of the image data.

The focus distance may be defined as the distance of the beam waist Wfrom a reference point of the particle optical system. The referencepoint may, for example, be a principle plane of the objective lens 30(shown in FIG. 2). A variation of the focus distance thereby causes avariation of the distance A. Hence, a variation of the focus distancemay lead to a different resolution of the image data which represent theportion B. A comparatively high resolution of the portion. B is achievedby a distance A of the portion B from the beam waist W of the primarybeam 201 being smaller than half of the depth of focus T.

The focus distance of the particle optical system 39 may be varied byvarying an excitation of the objective lens 30 (shown in FIG. 2).

FIG. 9 schematically illustrates how the surface model, of the structureis generated depending on a plurality of digital images 301, 302, 303according to an exemplary method. Each of the digital images 301, 302,303 has been generated by scanning the particle beam across at least aportion of the structure. The images 301, 302, 303 show a same portionof the structure. The digital images 301, 302, 303 have been acquired atdifferent focus distances of the particle optical system 39. Therefore,portions in the images 301, 302, 303, which represent a common portionof the structure, may have a different resolution. For simplicity ofillustration, only three digital images are shown in FIG. 9. However,the calculation of the surface model may be performed depending on morethan 5, more than 10, more than 20, more than 50 or more than 100digital images, which have been acquired at mutually different focusdistances. For example, the surface model may be generated depending onless than 500 or less than 200 digital images.

A plurality of image regions 310, 311, 312, 320, 321, 322 is selectedfrom the image data of each of the digital images 301, 302, 303. Forsimplicity of illustration, only six image regions are shown in each ofthe digital images 301, 302, 303. The plurality of image regions of adigital image may cover the whole, or substantially the whole digitalimage. The image regions 310, 311, 312, 320, 321, 322 of the digitalimages 301, 302, 303 are selected such that the image regions 310, 311,312, 320, 321, 322 may be divided into stacks, which show the sameportion of the structure.

In the embodiment, which is illustrated in FIG. 9, a first stack ofimage regions consists of the image regions 310, 311 and 312. Each ofthe image regions 310, 311, 312 of the first stack show a first commonobject portion. A second stack of image regions consists of the imageregions 320, 321 and 322. Each of the image regions 320, 321 and 322shows a second common object portion. The first common object portion isdifferent from the second common object portion. In the exemplaryembodiment, which is shown in FIG. 9, the first common object portion isadjacent and non-overlapping to the second common object portion.However, the first common object portion may partly overlap with thesecond common object portion. It is also conceivable that the firstcommon object portion and the second common object portion are neitheradjacent nor overlapping, but located at a distance spaced apart fromeach other. For simplicity of illustration, only six stacks of imageregions are shown in FIG. 9. For example, more than 100, more than10,000 or more than 10⁶ stacks of image regions may be generated fromthe digital images, wherein each of the stacks represents a differentportion of the structure. For example, less than 10⁹ stacks of imageregions may be generated from the digital images.

The stacks of image regions, which represent a common object region maybe determined by identifying object features, which appear in each ofthe digital images 301, 302, 303. For example, the identifying ofobject, features may comprise identifying edges, identifying adifference among image data and/or determining a frequency of image dataof an image region. The identifying of the object features may comprisesegmenting each of the digital images 301, 302, 303.

An image region consists of a group of pixels. An image region may havethe form of a square. For example, an image region may consist of 4times 4 pixels, of 8 times 8 pixels or of 10 times 10 pixels. An imageregion may be a pixel cluster, which has an irregular or non-symmetricalshape. An image region may consist of a single pixel.

The computer 70 (illustrated in FIG. 2) is configured to determine foreach of the stacks of image regions an image region which has thehighest resolution among all image regions in the respective stack andwhich is denoted herein as in-focus region. The in-focus region isselected from the image regions of the respective stack.

For example, from the image regions 310, 311 and 312, which form thefirst stack, the in-focus image region is selected. Furthermore, fromthe image regions 320, 321, 322, which form the second stack, a secondin-focus region is selected. Image region 311 is the in-focus region ofthe first stack and image region 322 is the in-focus region of thesecond stack.

Each of the image regions represents an X-coordinate value and aY-coordinate value in a plane perpendicular to the optical axis of theparticle optical system. The X-coordinate value and the Y-coordinatevalue of the image region 322 are schematically illustrated in FIG. 9.Furthermore, the focus distance at which the image data of an imageregion have been acquired, represents a Z-coordinate value of acoordinate axis, which is oriented parallel to the optical, axis of theparticle optical system.

The X-coordinate values, Y-coordinate values and Z-coordinate values ofall in-focus image regions represent a surface model of the structure.

FIG. 10 schematically shows a surface model 390 which has been generatedaccording to the method which has been described with reference to FIG.9. The surface model 390 is a two dimensional function, which assigns afunction value to discrete coordinate values in the X-Y-plane, whereinthe function value represents a coordinate value of the Z-coordinateaxis. Each of the function values of the two dimensional functioncorresponds to a focus distance of one of the determined in-focusregions within a stack. The discrete coordinate values in the X-Y-planecorrespond to the X-coordinate values and Y-coordinate values of thein-focus regions. The X-Y-plane corresponds to a plane, which isoriented perpendicular to the optical axis of the particle opticalsystem.

The computer 70 (illustrated in FIG. 2) is configured to store ameasurement location 340 relative to the surface model 390. For example,the computer 70 may be configured to determine which location of thesurface model of the structure represents a region at which the primarybeam impinges. The computer 70 is configured to assign image data of animage 341, which has been generated by scanning the primary beam at themeasurement location 340 to the stored measurement location 340. Theimage 341 may, for example, be a secondary electron image or an image,which has been generated by detecting back scattered electrons. Thestoring of the measurement location 340 may comprise storing ofX-coordinate values, Y-coordinate values and Z-coordinate values of themeasurement location 340.

This allows a user or an evaluation routine, of the computer todetermine based on the surface model 390, from which portions of thestructure high resolution images have already been generated.Furthermore, it is possible to interpret the image data of the image 341in dependence on the topography data of the surface model 390. Forexample, the surface portion, which is shown in image 341 may have asurface inclination, which is not recognizable in the image data of theimage 341. However, by storing the measurement location 340 relative tothe surface model 390, it is possible to recognize that the image dataof the image 341 represent a flank surface of the groove 342. Thereby,it is possible for the user or for the evaluation routine of thecomputer to determine a relationship or a dependence between the surfacetopography, which is represented by the surface model 390 and thedigital image data of the image 341. The image 341 may depend more oncompositional contrast than on topographical contrast. In particular,the digital image data of the image 341 may be generated depending ondetector signals of the detector for back scattered electrons. Thereby,to establish a relationship or a dependency between the compositionalcontrast of the image data of the image 341 and the surface topographyof the surface model 390.

FIG. 11 a schematically illustrates the generating of a surface model ofthe structure depending on detected particles according to a furtherexemplary embodiment. By scanning the primary beam, a plurality of imagegroups is determined. In the embodiment, which is illustrated in FIG. 11a, 12 image groups have been generated. Each of the image groupscomprises a plurality of digital images, which represent a same orsubstantially same portion of the structure. The images of the imagegroup, are generated at mutually different focus distances. A firstimage group 401 comprises the digital images 401 a, 401 b, 401 c,wherein for simplicity of illustration, only the pixel values of theimage 401 a are shown. Also for simplicity of illustration, only threedigital images of the image group 401 are shown. Similar to theexemplary embodiment, which is shown in FIG. 9, each of the image groupsmay comprise a plurality of digital images, in particular more thanthree digital images. A second image group 411 comprises the digitalimages 411 a, 411 b and 411 c. The images of all image groups representa structure, which comprises a portion of the surface of the objectholder 411 and a portion of the surface of the object 410, which areschematically illustrated in FIG. 11 b. The arrow, which is shown inFIG. 11 b schematically indicates an imaging direction VD of the digitalimages, which are shown in FIG. 11 a. The imaging direction VP isoriented parallel to the optical axis of the particle optical system.Each of the digital images, which are shown in FIG. 11 a, has beenacquired at a working distance of the particle optical system of 20millimeters. A side length of a field of view fv along an edge of thedigital images is 5 millimeters.

With a field of view of this size, it is not possible to image thecomplete top surface of the object 411 in a single scanning process.However, a surface model may be generated depending on the plurality ofimage groups of particle optical images, as shown in FIG. 11 a. Each ofthe image groups has been generated at a different position of thestructure relative to the objective lens. Each of the image groups yielda surface model. The surface models of the image groups are combined toform the surface model of the structure.

In the exemplary embodiment, which is shown in FIG. 11 a, the digitalimages of adjacent image groups show neighboring portions of thestructure, which overlap. For example, the portion, which is shown inimage 401 a overlaps with the portion which is shown in image 411 a.

Based on the images of each image group, image regions are generated, ashas been discussed with reference to FIG. 9. Thereby, for each of theimage groups, a surface model is obtained. The surface models ofneighboring groups overlap. Depending on the data values of the surfacemodel in the overlapping region, the surface models are combined to asurface model of the total structure.

Thereby, it is possible to generate a surface model of a structure bydetecting particles, wherein the structure has a greater extent measuredin a plane perpendicular to the optical axis than a side length vf of afield of view of an image of the particle optical system.

FIG. 12 schematically illustrates an alternative embodiment forgenerating a surface model depending on detected particles. A digitalimage 412 shows one of a plurality of images, from which image regions600 are generated. The image regions 600 show portions of the structure,which are located at a distance spaced apart from each other. In otherwords, image regions are neither adjacent nor overlapping. The imageregions 600 may consist of a plurality of pixels of between 1 and 8, orbetween 1 and 50, or between 1 and 500 or between 1 and 1,000 or between1 and 10,000 pixels. In the exemplary embodiment, as shown in FIG. 11,the image regions 600 are pixel clusters, each of which consist of 16pixels. For example, the first image region 500 consists of pixels 501,. . . 516.

Each of the pixel clusters is an isolated pixel cluster. In other words,each point of the structure, which is represented by the first imageregion, is located from each point of the structure, which isrepresented by a further image region, at least with a distance b. Oneof those further image regions is the image region 600. The distance bmay be a multiple of the diameter of a portion of the structure, whichis represented by a pixel of the pixel cluster. This diameter may bedefined as the sampling distance. The distance b may be greater than 10times, greater than 100 times or greater than 1,000 times the samplingdistance. The distance b may be less than 10,000 times the samplingdistance.

Accordingly, it is possible to calculate a surface model of thestructure within a comparatively short time. In particular, it isthereby possible that only a small portion of the structure has to bescanned by the primary beam and/or only image data from a comparativelysmall number of pixels have to be processed for generating the surfacemodel.

It is further conceivable, that one or more or all pixel clustersconsist of a single pixel. The pixel, represents a location at which theprimary beam is positioned at the structure. At this location, a focusdistance of the primary beam may be varied without scanning the surface.During the varying of the focus distance, particles are detected whichare generated by an interaction of the primary beam with the structure.Depending on the detector signal, it may be determined which focusdistance corresponds to the object distance, i.e. when a distancebetween the irradiated portion of the structure and the beam waist isless than half of the depth of focus. Thereby, it is possible togenerate a surface model of a structure in a very short time.

While the invention has been described with respect to certain exemplaryembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, the exemplary embodiments of the invention set forth hereinare intended to be illustrative and not limiting in any way. Variouschanges may be made without departing from the spirit and scope of thepresent invention as defined in the following claims.

The invention claimed is:
 1. A method of operating a particle beammicroscope, which comprises an objective lens having an object region,wherein the method comprises: detecting at least one of light rays whichemanate from a structure and particles which emanate from the structure,wherein the structure comprises at least one of at least a portion of asurface of an object and at least a portion of a surface of an objectholder of the particle beam microscope; generating a surface model ofthe structure depending on the at least one of the detected light raysand the detected particles; determining a position and an orientation ofthe surface model of the structure relative to the object region;determining a measurement location relative to the surface model of thestructure; and positioning the object depending on the generated surfacemodel of the structure, depending on the determined position andorientation of the surface model of the structure, and depending on thedetermined measurement location.
 2. The method according to claim 1,further comprising: generating a surface model of a microscope portionof the particle beam microscope; combining the surface model of thestructure and the surface model of the microscope portion to generate acombined surface model; and calculating a distance between the surfacemodel of the structure and the surface model of the microscope portiondepending on the combined surface model; wherein the positioning of theobject comprises monitoring the distance.
 3. The method according toclaim 1, wherein the determining of the position and orientation of thesurface model of the structure relative to the object region comprises:generating a digital image from at least a portion of the structure; andcomparing the surface model of the structure with the digital image. 4.The method according to claim 1, further comprising: determining asecond measurement location relative to the surface model of thestructure and relative to the measurement location; and repositioningthe object depending on the measurement location and the secondmeasurement location.
 5. The method according to claim 1, furthercomprising: generating a particle microscopic image, which represents atleast a portion of the measurement location; identifying a region of theparticle microscopic image; and adjusting at least one of a position andan orientation of the object depending on the identified region.
 6. Themethod according to claim 1, wherein the detecting of the at least oneof the light rays and the particles comprises detecting the at least oneof the light rays and the particles at a plurality of different focusdistances.
 7. The method according to claim 1, further comprising:generating digital image data, which represent at least a portion of thestructure depending on the at least one of the detected light rays andthe detected particles; wherein the generating of the surface model ofthe structure is performed depending on the digital image data.
 8. Themethod according to claim 7, wherein the generating of the digital imagedata comprises generating the digital image data from at least twodifferent imaging directions.
 9. A machine-readable medium having storedthereon a program code which, when loaded and executed in a computersystem, is adapted to perform the method according to claim
 1. 10. Amethod of operating a particle beam microscope, wherein the methodcomprises: detecting at least one of light rays, which emanate from astructure and particles, which emanate from the structure, wherein thestructure comprises at least one of at least a portion of a surface ofan object and at least a portion of a surface of an object holder of theparticle beam microscope; generating a surface model of the structuredepending on the at least one of the detected light rays and thedetected particles; generating a surface model of a microscope portionof the particle beam microscope; combining the surface model of thestructure and the surface model of the microscope portion to generate acombined surface model; determining a distance between the surface modelof the structure and the surface model of the microscope portiondepending on the combined surface model; and monitoring the distanceduring a positioning of the object.
 11. The method according to claim10, wherein the detecting of the at least one of the light rays and theparticles comprises detecting the at least one of the light rays and theparticles at a plurality of different focus distances.
 12. The methodaccording to claim 10, wherein the generating of the surface model ofthe structure further comprises: generating a plurality of stacks ofimage regions depending on the at least one of the detected light raysand the detected particles at the plurality of focus distances; whereinimage regions, which are part of a same stack of the plurality of stacksrepresent a same portion of the structure; determining for each stack ofthe plurality of stacks an in-focus region depending on the imageregions of the respective stack.
 13. The method according to claim 12,wherein each image region of at least a portion of the generated imageregions is an isolated pixel cluster.
 14. The method according to claim10, further comprising: generating digital image data, which representat least a portion of the structure depending on the at least one of thedetected light rays and the detected particles; wherein the generatingof the surface model of the structure is performed depending on thedigital image data.
 15. The method according to claim 14, wherein thegenerating of the digital image data comprises generating the digitalimage data from at least two different imaging directions.
 16. Themethod according to claim 10, wherein the detecting of the light rayscomprises: detecting of a laser beam, which has been reflected at thestructure.
 17. The method according to claim 10, wherein the generatingof the surface model of the structure comprises: generating a firstsurface model of a first portion of the structure in a first position ofthe structure relative to at least one of a light-sensitive imageacquisition device and the objective lens; generating a second surfacemodel of a second portion of the structure in a second position of thestructure relative to the at least one of the image acquisition deviceand the objective lens; and combining the first surface model and thesecond surface model.
 18. A machine-readable medium having storedthereon a program code which, when loaded and executed in a computersystem, is adapted to perform the method according to claim
 10. 19. Aparticle beam microscope system, comprising: an objective lens, havingan object region; an object holder which is configured such that anobject is mountable on the object holder; a positioning device, which isconfigured to adjust at least one of a position and an orientation ofthe object holder relative to the object region; a detecting device,which is configured to detect at least one of light rays, which emanatefrom a structure, and particles, which emanate from the structure,wherein the structure comprises at least one of at least a portion of asurface of the object holder and at least a portion of a surface of theobject; a computer, which is configured for signal communication withthe positioning device and the detecting device, wherein the computer isfurther configured to: generate a surface model of the structuredepending on the at least one of the detected light rays and thedetected particles; determine a position and an orientation of thesurface model of the structure relative to the object region; determinea measurement location relative to the surface model of the structure;and to position the object depending on the determined surface model ofthe structure, the determined position and orientation of the surfacemodel of the structure and the determined measurement location.
 20. Theparticle beam microscope system according to claim 19, wherein thecomputer is further configured to: generate a surface model of amicroscope portion of the particle beam microscope system; combine thesurface model of the structure and the surface model of the microscopeportion to generate a combined surface model; determine a distancebetween the surface model of the structure and the surface model of themicroscope portion depending on the combined surface model; and tomonitor the distance during a positioning of the object.
 21. Theparticle beam microscope system according to claim 19, wherein theparticle beam microscope system is configured to generate a digitalimage depending on the at least one of the detected light rays and thedetected particles, wherein the digital image represents at least aportion of the structure; and wherein the computer is further configuredto determine the position and the orientation of the surface model ofthe structure relative to the object region depending on a comparison ofthe surface model of the structure with the digital image.
 22. Theparticle beam microscope system according to claim 19, wherein thedetecting device is configured to generate digital image data dependingon the at least one of the detected light rays and the detectedparticles; wherein the computer is configured to determine the surfacemodel of the structure depending on the digital image data.
 23. Theparticle beam microscope system according to claim 22, wherein thedetecting device is further configured to generate the digital imagedata from at least two viewpoint positions.
 24. A particle beammicroscope system, comprising: an objective lens, having an objectregion; an object holder, which is configured such that an object ismountable on the object holder; a positioning device, which isconfigured to adjust at least one of a position and an orientation ofthe object holder relative to the object region; a detecting device,which is configured to detect at least one of light rays which emanatefrom a structure and particles, which emanate from the structure,wherein the structure comprises at least one of at least a portion of asurface of the object holder and at least a portion of a surface of theobject; a computer, which is configured for signal communication withthe positioning device and with the detecting device; wherein thecomputer is configured to: generate a surface model of the structuredepending on the at least one of the detected light rays and thedetected particles; generate a surface model of a microscope portion ofthe particle beam microscope system; combine the surface model of thestructure and the surface model of the microscope portion to generate acombined surface model; determine a distance between the surface modelof the structure and the surface model of the microscope portiondepending on the combined surface model; and to monitor the distanceduring a positioning of the object.
 25. The particle beam microscopesystem according to claim 24, wherein the detecting device is configuredto generate digital image data depending on the at least one of thedetected light rays and the detected particles; wherein the computer isconfigured to determine the surface model of the structure depending onthe digital image data.
 26. The particle beam microscope systemaccording to claim 25, wherein the detecting device is furtherconfigured to generate the digital image data from at least twoviewpoint positions.